1. Field of the Invention
The invention relates to angle data monitoring in a scanning-beam system generally, and more particularly to such a system for the so-called time-reference, scanning-beam microwave landing system.
2. Description of the Prior Art
The so-called scanning-beam microwave landing system of the time-reference type is sometimes known as "TRSB MLS" and that abbreviation will be used for convenience hereinafter. Such a system is prior art per se and has been described in the technical literature including the IEEE Transactions on Antennas and Propogation--Vol. AP-25, No. 3, May 1977. In that publication some aspects of the prior art in Time-Reference Scanning-Beam Microwave Landing Systems was described.
In the TRSB MLS system, a predetermined sector of space overlaying an airport runway or the extension thereof in space, is scanned by two narrow beams. The scan is oscillatory in nature, i.e. in one angular sense through the sector of space and then immediately in the other angular sense through the same sector of space. One beam is devoted to such a scanning action in the azimuth plane and the other in the elevation plane.
Equipment on an aircraft on landing approach can determine its angular position with respect to runway center by determining time between successive beam crossovers. Since the scan is reciprocating, i.e. back and forth or up and down for azimuth and elevation situation respectively, the closer the bearing of the aircraft is to the angular starting point of each cycle of first and second scan senses, the greater will be the time delay between first and second beam crossovers as detected on the aircraft. Similarly, an angle of approach near the turn-around in scanning sense produces a small time difference between beam crossovers.
The system to which the invention applies is in a sense, an open-loop system and therefore field monitors have been provided to check the function of beam-scanning angle versus time, since that function is critical to the accuracy of the system. For the purpose, field monitors, each containing a receiver and each connected back to the transmitting station by means of a cable have been provided. Knowing the angles of such monitors, circuits can be employed for comparing the actual instantaneous beam angle or the angular velocity of the scan or both. Such systems have usually been limited to two field monitors, since the associated cabling becomes burdensome, expensive and difficult to install if more such monitors are used.
The manner in which the present invention provides more complete monitoring throughout the sector of scan without encountering the expense and other difficulties of prior art monitoring techniques will be evident as this description proceeds.